1. Field of the Invention
The present invention generally relates to plastic injection molding machines and, more particularly, toward improved methods for exchanging molds for use in injection molding machines.
2. Description of Related Art
Injection molding machines have developed over the years to permit molding of a variety of parts, from small, simple plastic parts to rather large and complex plastic parts. For large parts, such as automobile bumpers, the injection molding machines and resulting injection molding dies are correspondingly large and heavy.
Due to the relatively short mold cycle time as compared to the rest of the factory, when using such large injection molding machines in a modern manufacturing environment, it is common to mold a particular part for a period of time with the first set of dies and then to exchange the first set of dies with a second set of dies to permit a different part to be molded. By switching the dies, the number of parts made with a single molding machine is increased. However, the die changing process has, in the past, been a time consuming operation requiring a significant amount of manual labor, and has resulted in a significant amount of down time for the machine. The manual labor involved in changing the dies is the result of the fact that each of the dies has a series of electrical, hydraulic, and cooling water connections that must be first unplugged from the first set of dies in order to permit the first set of dies to be removed from the machine. Thereafter, after the second set of dies is installed in the machine, these electrical, hydraulic, and cooling water connections must again be connected to the second set of dies. Since the number of connections may be between about 10-15, there is a significant amount of time involved, and is rather complicated for the operator.
Such large injection molding machines include an injector, a die assembly comprising a fixed die and a movable die, and die loading/unloading assembly including a movable mold transfer cart, and a part removal apparatus. The movable die is secured to a platan that is movable relative to the fixed die between a closed position and an open position.
The injector receives solid palletized or granular plastic material, heats and melts the plastic material, and pressurizes and injects the melted plastic into the die assembly. A screw type injector may be used in this regard. The die assembly receives the injected melted plastic and holds the plastic material until it solidifies, at which point the dies are moved apart to reveal the part. Ejectors are preferably built into the movable die, and serve to separate the molded part from the movable die. The part removal apparatus grasps the part, removes the part from the dies, and transports the part away from the dies.
The die loading/unloading assembly is used to replace or exchange dies in the machine, when necessary. In order to exchange a first set of dies in the machine with the second set of dies, which are stored on the mold transfer cart, the first set of dies are slid out of the machine and onto the transfer cart by the die loading/unloading assembly. Thereafter, the transfer cart is translated laterally to align the second set of dies with the opening in the machine, and then the second set of dies are slid into the machine by the die loading/unloading assembly.
More specifically, steps performed in a mold change sequence according to the prior art are illustrated in FIG. 1. First, the hoses and cables are disconnected. This step takes an inordinately large period of time because each of the dies has a plurality of hoses and cables connected thereto, and each of the hoses and cables must be individually separately disconnected and moved out of the way. Thereafter, the molds are unclamped, and then the rear machine door is opened. The molds are carried out by the die loading/unloading assembly, which conventionally includes a drive roller and idle rollers built into the machine and the transfer cart. Thereafter, the rear door is closed, and then the transfer cart is moved to place the second set of molds into alignment with the rear door opening. Simultaneously, a data conversion is performed to adjust for the change from the first die set to the second die set. The platan is positioned to receive the second mold set, and then the rear door is opened. The platan positioning is performed after the data conversion, but partially simultaneously with the movement of the transfer cart. The rear door is opened after the platan is positioned, and then the die loading/unloading assembly carries the second set of dies into the machine.
Once the second set of dies are inside the machine, the rear door is closed. Once the door is closed, the dies are clamped into place, a mold thickness step is performed, and the hoses and cables are reconnected. Mold thickness refers to calibration of the machine to the size of the dies to establish zeroes or set point positions. As when the hoses/cables are disconnected, reconnecting the hoses and cables takes and inordinately large period of time. When reconnecting the hoses and cables, care must be taken to make sure that the correct connections are made. Thus, locating the hoses/cables, and correctly reconnecting them to the mold takes a great amount of time.
As will be appreciated, the vast majority of the prior art mold change steps are performed sequentially or serially. Therefore, apart from speeding up any of the steps, there is little that can be done to improve the conventional serial operation of the prior art method and, more specifically, little that can be done to reduce the length of the prior art mold change method.
With reference to FIG. 2, a prior art machine operating method surrounding the mold change sequence of FIG. 1 is disclosed. As will be seen, the machine operating method also employs a generally sequential operation. Following production of the last part using the first set of dies, the R/B home is performed wherein the part removal apparatus is moved to a pre-programmed position by use a pendant control mechanism. Thereafter, the screw unit of the injector assembly is retracted, and then rust protection is applied to the first set of dies. Following application of the rust protection, the mold is closed and the cores are drained. Thereafter, the first set of dies is exchanged with the second set of dies, as discussed hereinbefore with reference to FIG. 1. Once the second set of dies are installed in the machine, the safety drop bar is set whereby the safety bar is moved into a position by a servo-driven screw to place a mechanical stop in place to prevent the movable platan from advancing forward anytime the machine front door is in the open position. Thereafter, an operational check, which is a check performed by the operator to visually confirm hydraulic units are functioning properly, is performed, and the screw unit is advanced. Since the R/B attachment change, wherein the part grasping head or chuck has been replaced with a part grasping head or chuck adapted to receive the new part, has been previously performed, the dies are monitored until plastization or melting of the plastic pellets occurs and then parts can begin to be made. As noted before with regard to the mold changing sequence, since the steps in the overall machine operating method associated with the machine operation surrounding the mold change are sequential or serial in nature, it is difficult or impossible to significantly reduce the time required. This has been an often-criticized problem with large injection molding machines that the industry has tolerated for years.
Since the second set of dies are cold when they are inserted into the machine, the second set of dies must be heated up to a molding temperature before good parts can be produced. Typically, the second dies are heated by electrical heating means built into the mold. When the plastic within the mold becomes liquid, which may take up to about 30 minutes, molding may begin. However, due to required normalization of the dies, it is commonly necessary to make a series of parts, sometimes up to a dozen parts, before good parts are made. Therefore, in addition to a great loss of time, the conventional mold-changing technique results in significant amount of waste.
As will be appreciated by those skilled in the art, machine down time resulting from die changes reduces the production of the machine in a given time period, and needs to be minimized to increase the productivity and efficiency of the manufacturing operation.
Therefore, there exists a need in the art for a method of reducing the downtime associated with mold change. Moreover, there exists a need in the art for speeding connection and disconnection of hydraulic and electrical connections during a mold change.